Photoresist compositions are used in microlithography processes for making miniaturized electronic components such as in the fabrication of computer chips and integrated circuits. Generally, in these processes, a thin coating of a film of a photoresist composition is first applied to a substrate material, such as silicon wafers used for making integrated circuits. The coated substrate is then baked to evaporate any solvent in the photoresist composition and to fix the coating onto the substrate. The baked and coated surface of the substrate is next subjected to an image-wise exposure to radiation.
This radiation exposure causes a chemical transformation in the exposed areas of the coated surface. Visible light, ultraviolet (UV) light, electron beam and X-ray radiant energy are radiation types commonly used today in microlithographic processes. After this image-wise exposure, the coated substrate is treated with a developer solution to dissolve and remove either the radiation-exposed or the unexposed areas of the photoresist.
There are two types of photoresist compositions, negative-working and positive-working. When positive-working photoresist compositions are exposed image-wise to radiation, the areas of the photoresist composition exposed to the radiation become soluble in a developer solution (e.g. a rearrangement reaction occurs) while the unexposed areas of the photoresist coating remain relatively insoluble to such a solution. Thus, treatment of an exposed positive-working photoresist with a developer causes removal of the exposed areas of the photoresist coating and the formation of a positive image in the coating, thereby uncovering a desired portion of the underlying substrate surface on which the photoresist composition was deposited. In a negative-working photoresist the developer removes the portions that are not exposed.
The trend towards the miniaturization of semiconductor devices has led both to the use of new photoresists that are sensitive to lower and lower wavelengths of radiation, and also to the use of sophisticated multilevel systems to overcome difficulties associated with such miniaturization.
High resolution, chemically amplified, deep ultraviolet (100-300 nm) positive and negative tone photoresists are available for patterning images with less than quarter micron geometries. There are two major deep ultraviolet (uv) exposure technologies that have provided significant advancement in miniaturization, and these are lasers that emit radiation at 248 nm and 193 nm. Examples of such photoresists are given in the following patents and incorporated herein by reference, U.S. Pat. Nos. 4,491,628, 5,350,660, EP 794458 and GB 2320718. Photoresists for 248 nm have typically been based on substituted polyhydroxystyrene and its copolymers. On the other hand, photoresists for 193 nm exposure require non-aromatic polymers, since aromatics are opaque at this wavelength. Generally, alicyclic hydrocarbons are incorporated into the polymer to replace the etch resistance lost by eliminating the aromatic functionality. Furthermore, at lower wavelengths the reflection from the substrate becomes increasingly detrimental to the lithographic performance of the photoresist. Therefore, at these wavelengths antireflective coatings become critical.
The use of highly absorbing antireflective coatings in photolithography is a simpler approach to diminish the problems that result from back reflection of light from highly reflective substrates. Two major disadvantages of back reflectivity are thin film interference effects and reflective notching. Thin film interference, or standing waves, result in changes in critical line width dimensions caused by variations in the total light intensity in the resist film as the thickness of the resist changes. Reflective notching becomes severe as the photoresist is patterned over substrates containing topographical features, which scatter light through the photoresist film, leading to line width variations, and in the extreme case, forming regions with complete photoresist loss.
The use of bottom antireflective coatings provide the best solution for the elimination of reflectivity. The bottom antireflective coating is applied on the substrate and then a layer of photoresist is applied on top of the antireflective coating. The photoresist is exposed imagewise and developed. The antireflective coating in the exposed area is then typically etched and the photoresist pattern is thus transferred to the substrate. Most antireflective coatings known in the prior art are designed to be dry etched. The etch rate of the antireflective film needs to be relatively high in comparison to the photoresist so that the antireflective film is etched without excessive loss of the resist film during the etch process. There are two known types of antireflective coatings, inorganic coatings and organic coatings. However, both of these types of coatings have so far been designed for removal by dry etching.
Inorganic type of coatings include such films as TiN, TiON, TiW and spin-on organic polymer in the range of 30 nm, and are discussed in the following papers: C. Nolscher et al., Proc SPIE vol. 1086, p242 (1989); K. Bather, H. Schreiber, Thin solid films, 200, 93, (1991); G. Czech et al., Microelectronic Engineering, 21, p.51 (1993). Inorganic bottom antireflective coatings require precise control of the film thickness, uniformity of film, special deposition equipment, complex adhesion promotion techniques prior to resist coating, separate dry etching pattern transfer step, and dry etching for removal. Another very important aspect of dry etching is that the harsh etch conditions can cause damage to the substrate.
Organic bottom antireflective coatings are more preferred and have been formulated by adding dyes to a polymer coating solution or by incorporating the dye chromophore into the polymer structure, but these too need to be dry etched down to the substrate. Polymeric organic antireflective coatings are known in the art as described in EP 583,205, and incorporated herein by reference. It is believed that such antireflective polymers are very aromatic in nature and thus have too low a dry etch rate, especially relative to the new type of non-aromatic photoresists used for 193 nm and 157 nm. In addition, photoresist patterns may be damaged or may not be transferred exactly to the substrate if the dry etch rate of the antireflective coating is similar to or less than the etch rate of the photoresist coated on top of the antireflective coating. The etching conditions for removing the organic coatings can also damage the substrate. Thus, there is a need for organic bottom antireflective coatings that do not need to be dry etched especially for compound semiconductor type substrates, which are sensitive to etch damage.
The novel approach of the present application is to use an absorbing, photoimageable positive working bottom antireflective coating that can be developed by an aqueous alkaline solution, rather than be removed by dry etching. Aqueous removal of the bottom antireflective coating eliminates the dry etch rate requirement of the coating, reduces the cost intensive dry etching processing steps and also prevents damage to the substrate caused by dry etching. The absorbing bottom anfireflective coating compositions of the present invention contain a photoactive compound and a polymer, which on exposure to light of the same wavelength as that used to expose the top positive photoresist become imageable in the same developer as that used to develop the photoresist. This process greatly simplifies the lithographic process by eliminating a large number of processing steps. Since the antireflective coating is photosensitive, the extent of removal of the antireflective coating is defined by the latent optical image, which allows a good delineation of the remaining photoresist image in the antireflective coating.
The antireflective composition disclosed in EP 542 008, is based on highly aromatic polymers, such as novolaks, polyvinyl phenols, copolymers of polyvinyl phenol and styrene or alphamethyl styrene, etc. Furthermore, this antireflective coating is not photoimageable and must be dry etched. Planarizing coatings that can optionally contain absorbing components are known and have been used to planarize topographical features and also prevent reflections. Planarizing layers are fairly thick and are of the order of 1 or 2 microns. Such layers are described in GB 2135793, U.S. Pat. Nos. 4,557,797 and 4,521,274. However these layers must be either dry etched or removed with an organic solvent, such as methyl isobutyl ketone. In the semiconductor industry removal of coatings by aqueous solutions is greatly preferred over organic solvents.
Bilevel photoresists are known, as in U.S. Pat. No. 4,863,827, but require exposure of two different wavelengths for the top and bottom photoresists, which complicates the processing of the lithography.
There are many patents that disclose antireflective coating compositions but these coatings are all cured to be insoluble in an aqueous developer solution and must be removed by dry etching. U.S. Pat. No. 5,939,236 describes an antireflective coating containing a polymer, an acid or thermal acid generator, and a photoacid generator. However this film is completely crosslinked to make it insoluble in an alkaline aqueous developer solution. The film is removed by a plasma gas etch. Examples of other antireflective coating patents are U.S. Pat. Nos. 5,886,102, 6,080,530 and 6,251,562.
U.S. Pat. No. 4,910,122 discloses an aqueous developable antireflective coating, however the degree of solubility of the total film is controlled by the bake conditions. This antireflective coating is not photoimageable, and therefore, there are no clearly defined soluble and insoluble regions in the film. The dissolution of the antireflective coating is controlled by bake conditions and thus the antireflective coating is very sensitive to the developer normality and developing time, and also gives poor resolution. High normality developer and/or long develop times can cause excessive removal of the antireflective coating.
Another process for imaging photoresists using antireflective coatings is disclosed in U.S. Pat. No. 5,635,333, however, the antireflective coating is not developed at the same time as the photoresist.
U.S. Pat. No. 5,882,996 describes a method of patterning dual damascene interconnections where a developer soluble antireflective coating interstitial layer is used. The antireflective coating is formed between two photoresist layers and has a preferred thickness of 300-700 angstroms, refractive index of 1.4-2.0 and is water soluble. The antireflective coating is not photoimageable and there is no description of the chemistry of the antireflective coating.
An acid sensitive antireflective coating is disclosed in U.S. Pat. No. 6,110,653, where the antireflective coating is crosslinked by a heating step and is subsequently rendered water soluble in the presence of an acid. The antireflective coating described contains a water soluble resin and a crosslinker, but other components, such as dyes, photoacid generators or amine base may be added. In this invention the water soluble resin is crosslinked before exposure, and if the composition additionally contains a photoacid generator, then the resin is uncrosslinked prior to development.
The novel antireflective composition of the present invention relates to a photoimageable, aqueous alkali developable, positive-working antireflective coating that is imaged with the same wavelength of light as is used to expose the positive photoresist, and thus is imagewise exposed in a single processing step. It is further heated, and then developed using the same developer and at the same time as the photoresist. The combination of single exposure step and single development step greatly simplifies the lithographic process. Furthermore, an aqueous developable antireflective coating is highly desirable for imaging with photoresists that do not contain aromatic functionalities, such as those used for 193 nm and 157 nm exposure. The novel composition enables a good image transfer from the photoresist to the substrate, and also has good absorption characteristics to prevent reflective notching and line width variations or standing waves in the photoresist. Furthermore, the novel antireflective coating can be designed, by using the appropriate photosensitivity, to function as an antireflective coating at any imaging wavelength. Additionally, substantially no intermixing is present between the antireflective coating and the photoresist film. The antireflective coatings also have good solution stability and form thin films with good coating quality, the latter being particularly advantageous for lithography. When the antireflective coating is used with a photoresist in the imaging process, clean images are obtained, without damaging the substrate.